Note: Descriptions are shown in the official language in which they were submitted.
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BACKGROUND OF THE INYENTION
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- This invention relates to the cuPlng of tobacco leaf.
The temperature and humidity in any type of tobacco curer must be
properly controlled if the tobacco leaf is to be cured without
spoilage in the minimum of time, with the best possible weight
in cured leaf of top quality. The curing process is dependent
inter-alia on the humidity and on the temperature inside the curer.
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Generally the temperature inside the curer is manually controlled
according to empirical formulae. Such~processes work satisfactorily
but however are not genera11y optimal. It is impossible to increase
the temperature in the curer manually precisely as the leaf requires
.; heat. The result of not being able to set the correct temperature
at the correct time results in scorching or early dehydration of the
leaf, or in loss of weight.
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~arious solutions have~been proposed and déscribed in the literatu~e,
see the specifications of U.S.A. Patents Nos. 3 503 137, 3 545 455,
; 3 618 225j 3 624 917, 3 664 034, 3 927 683, 3 937 227, 4 178 g46,
4 192 323 and 4 206 554.
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- The second last of these patents discloses a method ~or bulk curing
tobacco in which the temperature conditions in a curing barn are
automatically controlled by heating the air being circulated through
the barn in a controlled manner to maintain a predetermined difference
in the dry bulb temperature of the air en~ering and leaving the curing
chamber. To achiewe this objective temperature sensors are located
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externally of the barn exposed to the inlet and outlét air flows
respectively. As emerges from the disclosure the actual
temperature maintained in the barn is largely dependent on the
ambient temperature i.e. the temperature prevailing outside
the ba~n. It follows that the curing process is also dependent
on the temperature externally of the barn.
- SUMMARY OF THE INVENTION
It is an object of the present invention to provide an improved method
of curing tobacco leaf by controlling the temperature accurately in
the curer as the leaf requires it.
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The invention provides a method of curing tobacco lea~ in a curer barn
which ;ncludes the steps of supplying conditioned air to the curer
barn so as to maintain~between an upper zone and a lower zone
inside the curer barn, a first differential of a physical
characteristic of the atmosphere inside the curer barn which is
responsive to the curing process.
Further according to the invention the first differential is maintained
for a first period which su bstantially corresponds to the per;od for
colouring the leaf.
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The method includes the further step after the first period of
maintaining a second differential of the physical characteristic
for a second period which substantially corresponds to the period for
drying the leaf.
In order to avoid shock effects to the leaf i.e. scalding or
excessive drying of the leaf when the first differential is changed
to the second different;al the method includes the step of effecting
a gradual change of the first differential to the second differential
between th~e first and second periods.
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The physical characteristic may be the humidity level prevailing
in side the curer barn.
Alternatively the physical characteristic is the te~Eerature
prevailing inside the curer barn.
Either basis i.e. humidity or temperature, ~unctions satisfactorily
in ~hat the curing process is dependent on the state of the leaf
and on the conditions prevailing inside the barn and is not influenced
by the ambient conditions i~e. the humidity or tem~erature outside the
barn. This has the advantage that the curing prsceeds in a controlled
and substantially predetermined manner and rate and is not subject
to the vacJaries of the wea~her.
In the tem~erature based s~stem the first differential is of the order
of 7~ C and the secona differential is from lo& to 2 ~C and is
adjustable.
~he invention also provides tobacco during apparatus which comprises
a curer barn, ~ans for generating a ~irst signal which is dependent
on the level of a physical chara~teristic of the atmosphere prevailing
in an uppeL zone in side the curer barn~ the physical characteris~ic
- being responsive to the curing process, m~ans for generating a second
si ~ which is depen~ent on the level of the physical characteristic
of the abmosphere prevailing in a lcwer zone inside the curer barn,
m~ans for ocrparing the first and second signals and for generating a
control signal which is dependent on the first and second signals, and
m~ans for utilizing the co~trol signal to maintain a predetermined
difference in the levels of the physical characteristic prevailing in
the upper and lower 20nes respectively.
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Fur.ther according to the invention the means for utilizing the
control signal controls the flow of conditioned air into:the curer
barn.
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The first and second signal generating means may include sensors
which are either temperature or humidity dependent.
, BRIEF DESCRIPTION OF THE DRAWINGS
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The invention is further described by way of example with reference to
~, the accompanying drawings in which: '
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Figure 1 is a schematic view of a curer barn-with temperature control
apparatus according to.the invention~
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Figure 2 is a circuit diagram of'the ~emperature control apparatus
of Figure l,~and ~ ;
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Figure.3 is.a graph,of .temperature versus time inside the curer barn.-
' DESCRIPTION OF A PREFERRED EMBODIMENT
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, F~i.gure l ;ll~stratesr,~ ~u-sin.g f'~ ,which defines a curer barn 12.an~- .
which has a false floor 14. ::.
Equipment which is ancillary to the curer barn includes aIheat.source
6~ 0r,-~ample an au-~oma~ic s-toker:or aAb~rner,lan optional...hè,at :r/)~ L~
: exchanger,l'8 and a fan 22. A temperature probe 24 is located in an
.~ .upper zone inside the curer barn and a second temperature probe 26 is
located in a lower zone inside the curer barn. The two probes generate
. - 20 ,electrical signals-at ampl~itude~s which are proportional::to the respective
-' : temperatures i,n the zonesj and the electrical signals are applied to a
. controller 28.- -
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Figure 2 illustrates a circuit diagram of the controller 28.
Platinum temperature aependent resistors 24A and 26A respectively
are used for the probes and these are connected with resistors R,
- in a bridge configuration. The currents passed by the resistors
24A and 26A are proportional to the temperatures in the upper and
lower zones and the voltages across the resistors are therefore
proportional to the temperatures. The output voltages of the
bridge are applied to a first comparator 36 and the output of this
comparator is applied to an inverting input of a second comparator
; 10 38. The`positive terminal of the comparator is connected to an
adjustable voltage source 40. The output of the comparator 38
" drives a power transistor,42 which is connected in series with a -
relay coil 44.
Figure 3 illustrates a graph of temperature versus time within the
- 15 curer barn. ~he~graph has an upper curve 46 which illustrates the
variation with time of the temperature of the lower probe 26, and
a lower curve 48 which illustrates the variation with time of the
temperature of the~upper probe 24.
The tobacco leaf is loaded into the curer barn with maximum moisture
~0 and is tightly packed in frames between the upper and lower zone probes.
A barn of average size when filled with good quality tobacco leaf can
easily hold up to 10000 litres of water. -
When the tobacco leaf is placed in the barn the voltage source 40
is adjusted to maintain the temperature differential between the two
;~ 2~, pro~es 24 and 26 at approximately ~71/2C as shown in Figure 3.
This is achieved in the following way. The signals produced by the
two probes 24 and 26 are applied to the comparator 36 which goes on or
off depending on the setting of the inputs of the comparator. The
voltage of the output signal of the comparator 36 is compared to the
voltage of the source 40. Depending on the relationship of these
two voltages the output signal of the comparator 38 is either high or
low and the coil 44 is either not energized or energized through the
transistor 42. The contacts of the coil 44 control a contactor of
the heat source 16 ~h~ich is in series with a high iimit thermostat
or humidistat ~not shown).
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When heat is called for by the control circuit the contactor is
closed and the heat source 16 is turned on. Additional fuel
~ r'; '' '' -~is-supplied to the heat source and air which is passed throug~ the
heàt exchanger 18 by means o~ the fan 22 is heated to a higher
temperature. The hot air is circulated through the barn by means
of the fan 22.
; The hot air which is blown into the curer barn throuyh the f~loor
14 rises through, and dries, the leaf in the barn.
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As water is~evaporated from the leaf and taken up by the air the
temperature of the air drops and a temperature differential is
established between the upper and lower~probes. I~ the temperature
of the air in the lower zone is too high ~hen the differential increases.
As the differential is dependent on the evaporation rate of the leaf
a high diffPrential indicates that the drying process is too rapid.
Conversely a low differential is indicative of a low evaporation rate
i.e. that the drying process shoul;d be accelerated.
The control circuit funotions to maintain the differential at a value
which has been empirically determined as being optimal, depending on
- , the stage of the curing. However the differential control is not
~ an absolute temperature d;fference imposed on the curer barn regardless
of the q~lities o~the leaf to be cured. The differential control is
inherently~dependent on the humid;ty level in the curer barn, and this
'''''''`i'S r dependent on the moi~ture conten~-of the leaf, the evaporation ~a~e~
of the leaf, and the temperature of the air introduced into the lower
zone.`
It follows that the ~ystem i.s~lone which is dependent on an empirically
determined temperature differential but that the control function is
automatically adapted to the qualities of the leaf to b cured.
urther, since;the~tempera~ure s~nsors are located inside the~urer
barn the control function is independent of the ambient conditions
i.e. temperature and hum;dity outside the barn. The control function
is therefore more precise and efficient.
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The initial temperature differential of approximately 71/2C is
maintained until the leaves have been properly colouredO Thereafter
as shown in Figure 3~the voltage source 40 is adjusted so as to
main!tain a greater maximum temperature differential between the two
probes., The second temperature differential is determined empirically
but generally is of the order of from lDC to 20C. The circuit
- functions in precisely the same way but in this case as the leaves aredrying more uniformly throughout the curer barn a greater quantity
of heat is called fQr by the curer in order to;maintain the temperature
differentiàl between the two probes. Cons`equently the average
:, temperature inside the curer-incre~ses gradually to a maximum of 75C
which is determined by means of the high limit thermostat. Eventually
the temperatures in the~upper and lower zones of the curer are
approximately the same and~the two curves 46 and 48 meet. At this
stage the leaves are completely dried.
An important aspect of the invention concerns the transition between
the yellowing and drying stages iOe. the increase of the differential.
In accordance with the invention this is effected gradually, at a rate
of between l/2C to 1l~2C per hour, by manually adjusting the control
of the variable voltagP source 40. This ensures that the leaf is not
"shocked" or s~alded by an abrupt temperature increase and ensures,
- , that the f-inal product is a high quality leaf.
In the method of the invention the leaf it;elf determines the rate at
which the temperature~in the curer barn advances. The upper zone
~ 5-- probe acts like a wet bulb -because;Qf-the eva,poration of water f,rQm "~ "
- ~, the lea~ which cools,it down. The resulting slow advance in
temperature cannot be emulated manually, ,~
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On colouring of;tobacGot although the air in the curer barn is circulated,
: no intake of fresh air, or exhausting of air, takes placej~ and the
humidity is approximately 90% RH. When the temperature, and consequently
the humidity, increases, a hum;distat causes a vent to be opened and
--- fresh air iis~drawn into ~he curer-b~rn-, and moisture is expelled from it.;The humidity is thereby reduced. As the moisture is reduced in the barn
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and expelled through the outlet ventilators5 the temperature of the
upper probe rises and the lower probe follows the upper probe. This
is the method of increasing the temperature in the curer.
The advantage of the invention lies in the fact that the control
equipment maintains the temperature in the curer barn at an optimal
value which ensures minimum curing time without spoilage and maximum
weight and best quality leaf. The reduced curing time in turn reduces
the fuel demand of the burner.
For example a curer barn fitted with the control equipment of the
- 10 invention required approximately 21 x 106 BTU's~and cured ripe tobacco
in 5 days. Previously, with conventional curing control equipment,
the curing time was 7 to B days and 36 x 106 BTU's were required.
- - ^ In addition to the general principles of the invention described thusfar
there are two important elements which must be taken into account of
during the curing process. The first~is that during the yellowing
stage the temperature inside the curer barn should not exceed a value
of from 36C to 38C. If this temperature limit is exceeded the
cellular structure of the leaf is destroyed and the quality of the leaf -
is reduced. The possibility of this happening is avoided by using a
temperature sensor which is located inside the curer barn to shut down
the heat source if the temperature reaches the preset limit.
This aspect emphasises that the curing process should be independent
of ambient conditions. If this is not the case the temperature inside
the curing barn fluctuates as the ambient temperature changes and, ~
although the temperature differential may be maintained, it is quite
possible for the temperature in the curing barn to exceed the preset
limit with a consequent deterioration in the quality of the leaf.
Closely related to this aspect is the fact that in a system which is
dependent on ambient conditions an increase in the external humidity
causes a decrease in the internal temperature, and vice versa. Both
of these effects can be harmful for, in the former case the leaf in the
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curer barn which is already dried tends to reabsorb moisture while, in
the latter case ? i f the temperature rise is rapid 5 the leaf is damaged.
Agaln it should be pointed out that these possibilities are eliminated in
the present invention which provides a system which is independent of
ambient conditions, for the curing process is determined by the condition
of the leaf itselfO
The second important element to be taken account of during the curing
process relates to an aspect which has been determined by the applicant
~-' through a gr~at deal of experimentation and which is best illustrated
by way of an example. Assume that the desired temperature differential
is 12C and that the temperature of the lower probe is 60C with the
' temperature of the upper probe being 48C. When the temperature at
the lower probe starts dropping below 60C the heat source is turned
on and warm air enters the curer barn, thereby causing the temperature
at the lower probe to increase. In accordance with the invention the
heat supply is turned off before the effect of the additional heat
manifests itself at the upper probe i.e. before the upper probe
temperature can increase. In practice this means that the temperature
differential of 12C' is maintained with a fine tolerance of approximately
' 20 -1/4C. Over a short interval therefore the upper probe temperature is
- ~ ~essentially constant at 48C and the lower probe temperature is between
60C and 60 l/40C. Eventually though the additional heat manifests
itself at'the upper probe and the temperature of this probe then
increases'slightly. The lowerprobe then follows suit.
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If this tolerance is not maintained the temperatures at the two'probes
increase simultaneously, although not at the same rate1 and an unstable
condition may result which leads to an excessive temperature rise and
consequent-leaf damage.
Finally it should be pointed out that the temperature differential, which
is determined empirically, is dependent on the desired temperature
gradient in the curer barn. For example,if the curer baPn holds three
layers of tobacco leaf the d;fferential may be fixed at 12C. However
if the barn holds't~o layers of similar tobacco leaf the differential
would-be of the order of 8C so that the temperature gradiant of
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3~ approximately 4C per layer is maintained.
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The invention has been described with reference to a temperature
based system. The principles of the invention are however, equally
applicable to a humidity based system which in a manner anologous to
- - that described, employs upper and lower humidity sensors. These
sensors replace the temperature sensors 24A and 26A but in other
- respects the principle of operation is unaltered and the same benefits
result. This is because the temperature prevailing in a given zone
of the barn is dependent on the humidity level in the same zone and
this, in turn, is dependent on the moisture in the leaf and the curing
rate.
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